One type of thermal printer employs a dye-donor element placed over a dye-receiver element. The two elements together are moved past a print head having a plurality of very small heat "sources". When a particular heating source is energized, thermal energy from it causes a small dot or pixel of dye to transfer from the dye donor element onto the receiver element. The density of each dye pixel is a function of the amount of energy delivered from the respective heating source of the print head to the dye donor element. The individual pixels are printed in accordance with image data. All of the dye pixels thus formed together define the image printed on the receiver element.
Because light from a laser can be focused to an ultra-fine, intense spot of heat energy and can be modulated at very high speed, lasers (e.g., small, relatively inexpensive diode lasers) are now the preferred heating sources for printing the dye pixels in more advanced thermal printers. In the case where pixels are printed at very fine pitch on very closely spaced lines (e.g., 1800 lines per inch and 1800 pixels per inch), hundreds of millions of pixels are used in printing a page size picture. It is costly at present to provide an individual laser for each line of pixels across the width of a page being printed. For example, a 10 inch wide page would require 18,000 lasers, along with their respective drive circuits. On the other hand, using only one laser and scanning in sequence the lines across a page to print an image pixel by pixel is a very much slower operation than when multiple lasers are used.
In U.S. patent application Ser. No 451,655, filed Dec. 18, 1989, now U.S. Pat. No. 5,164,742, entitled "Thermal Printer" and assigned to an assignee in common with the present patent application, there is disclosed a thermal printer employing a plurality of lasers for printing a like plurality of lines of print pixels at the same time. This thermal printer produces full color pictures printed by thermal dye transfer in accordance with electronic image data corresponding to the pixels of a master image. The pictures so produced have ultra-fine detail and faithful color rendition which rival, and in some instances exceed in visual quality, large photographic prints made by state-of-the-art photography. This new thermal printer is able to produce either continuous-tone or half-tone prints. In the continuous tone mode, the ultra-fine printed pixels of colored dye have densities which vary over a continuous tone scale in accordance with the image data. On the other hand in the half-tone mode, the ultra-fine print pixels which define the picture are formed by more or fewer subpixels of dye such that a greater fraction of the area of each pixel is darkened or remains undarkened in order to appear to the eye as having greater or lesser density and thus simulate a continuous tone scale. Half-tone offset printing is widely used in printing and publishing.
The human eye is extremely sensitive to differences in tone scale, to apparent graininess, to color balance and registration, and to various other incidental defects (termed "printing artifacts") in a picture which may occur as a result of the process by which the picture is reproduced. Thus it is highly desirable for a thermal printer such as described above, when used in critical applications, to be as free as possible from such printing artifacts.
The thermal printer described in the above-mentioned U.S. Patent Application has a rotating drum on which can be mounted a print receiving element with a dye donor element held closely on top of it. The two elements are in the form of thin flexible rectangular sheets of material mounted around the circumference of the drum. As the drum rotates, a thermal print head, with individual fiber optic channels, projects multiple laser light beams in closely spaced, ultra-fine light spots focused on the dye donor element. Simultaneously the print head is moved in a lateral direction parallel to the axis of the drum so that with each rotation of the drum multiple lines (termed a "swath") of subpixels are printed on the receiving element. The pixels are printed in accordance with image data applied to the electronic driving circuits of the respective laser channels. There are as many image lines in a swath as there are laser channels (e.g., 12 lines with a lateral spacing of 1800 lines per inch), and there are as many swaths as required to print an image or picture of a given page width. It has been found with such a printer, in the absence of expensive corrective measures, that there may be produced visually noticeable printing artifacts in the picture which impair its quality.
In the kind of thermal dye-transfer imaging described above, there is employed a dye donor element in the form of a thin sheet of material having a thermally reactive dye on one surface. Such a donor element is disclosed in U.S. Pat. No. 4,973,572 and assigned to an assignee in common with the present patent application. The donor element is placed with its dye coated surface closely adjacent (e.g., about 8 micrometers distant) to a receiver element (e.g., a suitable sheet of paper). Then the donor element is "scanned" by each laser beam focused on the back of the donor element to a very small spot of light (e.g., about 7 micrometers diameter). As explained in U.S. Pat. No. 4,973,572, the dye donor element contains an infrared light absorbing compound which generates heat from the laser spots and causes subpixels of visible dye carried by the donor element to transfer to the receiver element to produce an image. As each laser spot is linearly scanned along the donor element, each laser is electronically modulated at very high frequency to provide greater or lesser heat energy in the focused light spot. The thermal energy in a respective light spot passing through the donor element causes the dye over the area of the spot to vaporize to a greater or lesser degree depending on the heat energy content of the laser light spot. The dye thus removed in the area of the light spot transfers as a dot or pixel of dye and is deposited onto the receiver element. The density of such a transferred dot of dye is a function of the total thermal energy absorbed through the donor element into the dye at the light spot.
It has been found that the density of a pixel of dye in a thermal printer of this kind after being printed on the receiver element may not be properly related to the amount of energy provided by its particular laser beam. For example, the thermal energy applied instantaneously to a particular spot by its respective laser beam may also have unwanted or excess heat energy added to it by thermal migration of energy within the dye donor element from a closely spaced laser light spot produced by an adjacent channel being operated at the same time though independently modulated. As a result of this unwanted thermal interaction amongst the independent laser channels, densities of some pixels of the printed image may not be exact reproductions of the densities of the master image. This results in "printing artifacts" such as dark streaks termed "banding", and in a degradation of the visual quality of the printed image, especially when viewed critically.
Various different thermal printing system using pre-heating in one form or another of a dye donor element prior to its being energized by a heat source in printing a dye pixel onto a receiver element have been tried in the past. Increases in printing speed and reductions in power necessary for printing have been claimed. But nonetheless, problems of "printing artifacts" in multiple laser printers and of obtaining the highest fidelity of reproduction of a master image have remained. The present invention provides an efficient and cost effective solution to these problems.